I would replace the entire line to your house from the meter with SCH40 PVC or Copper Type K. Check local codes and depth requirements. You can do the work yourself since it is your property. You still probably will need to pull permits.
Also you need to check local codes to make sure you dont need a separate meter for your irrigation that you want to install. Most areas wont allow you to tie in to your "house meter"
If it is allowed and you want to go ahead and do it;
You can rent pipe die threaders that is a hand held ratcheting type threader.
You will have to cut a section of the metal pipe out and thread both ends and then install the T and rejoin the ends together. Be sure to install a QUALITY shut off valve underground before the backflow preventer for service of the irrigation system.
I guess the PITFALL or WARNING would be that if the steel pipe has substantially aged it could be brittle and unable to thread once you cut your section out to make the T. So thats why im recommending you replace the entire line that feeds your house. Also galvanized piping collects corrosion on the inside of the pipe and gets worse over time, eventually stopping the water flow.
ALSO, if you decide to go ahead and replace the ENTIRE line, I would have someone with a camera run down your sewer line from your house to the street and check that pipe integrity as well. It would be convienent to replace both lines at once, since they were probably installed in the same ditch up to your house.
Last edited by koster_irrigation; 01-20-2013 at 12:13 PM.